For the ground wave television broadcasting, a variety of broadcasting systems are currently available. For such ground wave television broadcasting, the M(NTSC) system is generally used in Japan and the United State. In this M(NTSC) system, the number of scanning lines is set to 525, the channel band width is set to 6 MHz, and the frequency difference between the video carrier wave and the audio carrier wave is set to 4.5 MHz.
In Germany, the B-system (PAL) is adopted in the VHF (Very High Frequency) band width, whereas the G-system (PAL) is adopted in the UHF (Ultra High Frequency) band width. In the B-system, the number of scanning lines is set to 625, the channel band width is set to 7 MHz, and the frequency difference between the video carrier wave and the audio carrier wave is set to 5.5 MHz. In the G-system, the number of scanning lines is set to 625, the channel band width is set to 8 MHz, and the frequency difference between the video carrier wave and the audio carrier wave is set to 5.5 MHz. As in this case of Germany, there are cases where different broadcasting systems are adopted even within the same country.
Particularly, the European region where many countries are adjacently located is in an environment of mixed broadcasting systems are mixed, including not only the above-explained broadcasting systems but also the I-system with the frequency difference between the video carrier wave and the audio carrier wave of 6.0 MHz, and the D-system and the K-system with the frequency difference between the video carrier wave and the audio carrier wave of 6.5 MHz.
On the other hand, in the television broadcast receiver, a receiving high frequency signal (hereinafter referred to as a receiving RF signal) is once converted into an intermediate frequency signal (hereinafter referred to as an IF signal). The IF signal is then separated into a video signal component (hereinafter referred to as a VIF signal) and an audio signal component (hereinafter referred to as a SIF signal). Then, by mixing a local oscillating signal (hereinafter referred to as an LO signal) with the SIF signal, it is possible to be demodulated into an audio signal. This is based on such characteristic that the lower is the frequency, the simpler is the structure of a wave detector, and for this reason, almost all the wave detectors are arranged so as to demodulate an audio carrier frequency signal after once being converted into a SIF signal.
FIG. 5 is a block diagram showing an electrical structure of a television broadcasting receiver 1 applicable to the NTSC and PAL broadcasting systems for demodulating using such SIF signal. The receiving RF signal as input from an antenna 2 is first input to an input turning circuit 3 composed of a band pass filter, where only a signal component as desired is extracted. Then, after being amplified by a frequency amplifying circuit 4, the RF signal is input to an inter-stage turning circuit 5 composed of a band pass filter, where further unnecessary signal component is extracted. This RF signal is then input in a mixer 6 to be mixed with an LO signal generated in a local oscillating circuit 7. The IF signal is then subjected to frequency conversion to obtain the IF signal.
The foregoing IF signal is input in common between SAW filters 8 and 9 applicable to the NTSC broadcasting system and the PAL broadcasting system, and the video signal component (VIF signal) and the audio signal component (SIF signal) are extracted. For these SAW filters 8 and 9, a SAW filter for video and a SAW filter for audio may be adopted separately. Then, depending on which of the NTSC broadcasting and the PAL broadcasting is to be received, from either the SAW filter 8 or the SAW filter 9, the VIF signal can be applied to a video IF amplifying circuit (VIF AMP) 10, and the SIF signal can be applied to an audio IF amplifying circuit (SIF AMP) 11.
The VIF signal as amplified in the video IF amplifying circuit 10 is subjected to video wave detection to be formed into a video signal in a video wave detecting circuit (Video DET) 12, and the video signal is output after being amplified in a video amplifying circuit (Video AMP) 13.
The SIF signal as amplified in the audio IF amplifying circuit 11 is subjected to wave detection (frequency conversion) in an audio demodulating circuit (QIF DET) 14 to be formed into an SIF signal of 4.5 MHz in the case of adopting the NTSC (M-system), and an SIF signal of 5.5 MHz in the case of adopting the PAL (B-system or G-system) respectively. Then, after being subjected to the FM wave detection in an FM detecting circuit (FM DET) 15, the SIF signal is output as an audio signal.
For the receiver 1 having the foregoing structure, a particular broadcasting system is set in each country; however, the broadcasting system becomes of more commercial value by setting such that a television program being broadcasted in one of the neighboring countries can be seen in other neighboring countries. However, in order to receive television broadcasting of all the broadcasting systems, the audio demodulating circuit 14 of an extremely complicated structure is needed.
At present, many Integrated Circuits (ICs) are developed for the described receiver 1, and therefore the foregoing problem can be solved by the 1-chip IC (many systems can be received by one IC). The problem of the frequency difference between the video carrier wave and the audio carrier wave as the most series problem in receiving the television programs in a variety of broadcasting systems may be resolved by adopting the PLL (phase-locked loop) for the audio demodulating circuit 14.
FIG. 6 shows one example of the audio modulating circuit 14 when adopting the PLL. In the generally used television broadcasting, a SIF signal obtained by FM demodulating an audio signal is superimposed in a video signal, and the resulting signal as superimposed is further modulated before being transmitted. Therefore, the frequency of the SIF signal is given by a frequency difference between the video carrier wave and the audio carrier wave. For example, in the M-system, the frequency difference is set to 4.5 MHz, whereas in the B-system, the frequency difference is set to 5.5 MHz.
In the audio demodulating circuit 14, a sine wave LO signal having the same frequency as the SIF signal is prepared by a voltage control oscillator (VCO) 21, and is mixed with a SIF signal in a mixer 22. Then, by taking the difference between the SIF signal and the sine wave LO signal of the same frequency, an audio output is extracted to a FM wave detecting circuit 15. The LO signal is a complete sine signal, which is a clear signal basically without being modulated nor containing noise.
Therefore, following the changes in frequency of the SIF signal, it is required to accurately prepare the LO signal having the same frequency. In response, the phase differences between the SIF signal and the LO signal are mutually compared in the phase comparator 23, and the control voltage corresponding to the phase difference is applied to the VCO 21. As a result, the frequency of the LO signal can be set to the frequency of the SIF signal. These VCO signal 21 and the phase comparator 23 prepare PLLs, and the LO signal corresponding to SIF signals of a variety of frequencies.
However, required performances cannot be ensured merely by adopting the PLL circuit for the audio demodulating circuit 14. Therefore, in its application to high quality model, it is required to control the audio demodulating circuit 14 by a control signal from an external section of the audio demodulating circuit 14. This is to correspond to the neighboring interferences regulated by the CENELE standard (European Committee for Electrotechnical Standard). In the following, the neighboring interference will be explained in reference FIGS. 7(a) to 7(c), and FIGS. 8(a) and 8(b).
FIGS. 7(a) to 7(c) are diagrams, each illustrating a spectrum of the IF signal. Specifically, FIG. 7(a) illustrates spectrums of the B-system and the G-system, FIG. 7(b) illustrates a spectrum of the I-system; and FIG. 7(c) illustrates a spectrum of the D-system and the K-system, wherein “C” indicates a carrier frequency, “P” indicates a central frequency of the video signal, and “S” indicates a central frequency of an audio signal.
A mechanism for generating a neighboring interference will be explained in reference to FIGS. 8(a) and 8(b). FIG. 8(a) illustrates a spectrum of the B-system, and FIG. 8(b) illustrates a spectrum of the D-system. The video signal is AM modulated with the point P of 38.9 MHz as a central frequency, and the side band on the upper side is cut in relation to the band restriction of the channel, and only the side band on the lower side appears.
Then, the central frequency P′ of the video signal in the adjacent channel is 31.9 MHz in the B-system as 7 MHz in the channel band width is separated; and is 30.9 MHz in the D-system as 8 MHz in the channel band width is separated. For this reason, with the structure wherein the SIF signal of 32.4 MHz 6.5 MHz separated passes, i.e., the frequency difference between the video carrier wave and the audio carrier wave in the D-system, upon receiving the B-system broadcasting, based on the video signal of the adjacent channel of 31.9 MHz, the interference generates in the audio signal. For this reason, as illustrated in FIG. 9, the audio demodulating circuit provided with a plurality of trap members 32 and 33 in the IF signal line 31 may be used.
However, as indicated with a reference numeral 34 in FIG. 1, only by merely adopting a plurality of trap members 32 and 33, as indicated with a reference numeral 34 in FIG. 10, a sufficient selectivity cannot be ensured with respect to the selectivity of the trap members 32 and 33 shown by the reference numerals 35 and 36, and in some case, a necessary signal may be attenuated.
As a typical prior art to solve the foregoing deficiencies, Japanese Unexamined Patent Application No. 9-298754/1997 (Date of publication: Nov. 18, 1997). In this prior art, by switching between the trap members 32 and 33, it is possible to use only an appropriate trap member, and mutual interferences can be obtained.
However, in this prior art, in response to the user operation, it is necessary to control (switch the broadcasting system) the circuit based on the control signal as input from the external section, and the user need to perform troublesome tasks each time switching the local station.
The control signal as input from the described external section is an I/O (input/output interface) signal as generated in a microcomputer in an external section, and a circuit is required for converting the I/O signal into a signal which permits each circuit in the audio demodulating circuit becomes operable. Specifically, it is necessary to separately provide a switching circuit which is set to be 5V with the control signal logic of “1”, and is set to be 0V with the control signal logic of “0”, thereby presenting the problem in that a variable and a large scale circuit is needed.